1. Field of the Invention
The invention relates to the field of iron and steelmaking, and the refining of other metals, and particularly relates to a process of recovering, from a blast furnace operation, high-grade metal having a substantial amount of impurity, and then removing the impurity to allow the metal commercially to be used.
2. State of the Art
In the normal operation of iron blast furnaces, liquid iron is continuously collected in the hearth, and liquid slag is simultaneously forming and collecting as a separate, completely immiscible layer on top of the iron. As used herein, the term "slag" means any material having a high degree of impurities. "Iron slag," the precise composition of which can vary substantially, is produced in the production of blast furnace iron and typically will have an approximate composition of 38 percent by weight of calcium oxide, 13 percent of magnesium oxide, 36 percent of silicon dioxide, 10 percent of aluminum oxide, 1.5 percent of sulfur, and minor amounts of other metallic oxides.
In all iron blast furnaces, the liquid iron slag must be flushed out every few hours. This iron slag is usually removed by allowing it to flow through a tap hole, known as a "slag notch" or "cinder notch," which is located high in the blast furnace at approximately the normal level of the slag. The molten iron slag is thus allowed to run out of the blast furnace every few hours, in order to prevent accumulation of undue amounts of the slag. However, because iron is constantly migrating through the slag layer during the operation of the furnace, the pouring of the iron slag will result in a concomitant loss of approximately 1 percent by weight of the total amount of iron produced by the blast furnace.
A typical blast furnace will produce approximately 3,000 tons of iron per day, and large blast furnaces may produce as much as 10,000 tons of iron per day or more. It is therefore quite common to experience a loss of from about 30 to 100 tons per day of iron with the removal of the iron slag from the blast furnace. Iron, with an acceptably low level of impurities contained therein, as is found in normal blast furnaces, has a very substantial value. Thus, such losses of up to 100 tons of iron per day are obviously substantial and are desired to be minimized or eliminated completely.
In the common prior art process, the molten blast furnace iron slag is conveyed into one of two large, walled-in slag pits located right next to the furnace, where it is allowed to solidify and cool. The slag pit contains primarily slag, having, however, substantial irregular-shaped masses of iron of widely-varying size scattered throughout the slag, like raisins in a cake. Depending primarily on the quality of the raw materials entering the blast furnace, i.e. iron ore, coke and limestone, the amount of iron slag produced per ton of iron will vary substantially, and the amount of iron distributed throughout the slag pit is usually in the range from about two to five percent by weight of the total slag pit contents. The iron distributed through the iron slag in the slag pit is the same as the regular blast furnace iron product, having a relatively low level of impurities and high economic value, if it can be separated adequately from the surrounding iron slag.
When the slag pit being worked is full, the slag from the blast furnace is then channelled to the other slag pit, which is then empty, having earlier been emptied by power shovels and trucks while the other slag pit was being filled. These power shovels and trucks are moved over to the full pit, after a period of time has elapsed to allow solidification of the iron slag, and the trucks are filled by the power shovels with large chunks of slag from the slag pit, which are transported to a slag processing facility, generally located nearby.
In the slag processing facility, the large chunks of slag, having dimensions of up to several feet, are impacted by heavy wrecking balls, to crush the slag into pieces generally of up to two feet in maximum dimension. In the past, when the slag processor had broken the iron pieces contained in the slag down to a size which could be accommodated by an open hearth furnace charging pan, this iron-containing material, called blast furnace slag iron herein, was separated from the pieces of iron slag by use of an electromagnet, to be recycled to an open hearth furnace. These blast furnace slag iron pieces, are highly variable in their iron and iron slag content, but will frequently comprise about 60 to 80 percent of blast furnace iron, having a low sulfur content of typically about .03 percent by weight, and 20 to 40 percent of iron slag, having a sulfur content of about 1 to 2 percent by weight.
Because of the substantial time involved in producing steel using an open hearth furnace, due to the inherently long melting, refining and superheating cycle, the blast furnace slag iron pieces, which have a high sulfur content -- since there is 1 to 2 percent of sulfur in the slag -- were still reusable, and could be used in open hearth steel production. The considerable melting and refining time inherent in the operation of open hearth furnaces, allowed ample opportunity for all of the sulfur to be removed in the open hearth furnaces. Therefore, so long as open hearth furnaces were popular, no major problem was encountered in recycling blast furnace slag iron.
However, basic oxygen furnace steelmaking is steadily replacing basic open hearth steelmaking as the process of choice for converting blast furnace iron to steel. It is expected that, in a relatively short period of time, substantially all blast furnace iron in the United States will be refined to steel by the basic oxygen furnace method. Although the basic oxygen furnace process is vastly superior to the open hearth process in most ways, the basic oxygen furnace is inferior with respect to its flexibility in handling scrap metals, such as the blast furnace slag iron pieces described above. In particular, most basic oxygen furnace operators prefer to exclude these blast furnace slag iron pieces from their charges, because these pieces are usually considered to be too high in sulfur content for optimum efficiency steelmaking, due to their excess iron slag content. Unimproved blast furnace slag iron is not suitable for basic oxygen furnace processing, because of the relatively short residence time of the iron in the basic oxygen furnace. This residence time is usually not adequate to achieve sufficient desulfurization of the relatively high sulfur content blast furnace slag iron pieces. Therefore, in any integrated steel mill having only basic oxygen furnace steelmaking producton, the blast furnace slag iron pieces produced from the mill's blast furnace slag are commonly accumulated in large stockpiles as a low grade form of scrap, too good to haul to the dump, but not good enough to charge, as is, into the steelmaking basic oxygen furnace. This scrap has had a very low economic value in the past because, in addition to its unsuitability for basic oxygen steelmaking, it could not be utilized in any other sector of the ferrous industries, such as in iron foundries.
The process of this invention involves the use of a shot blasting and tumbling machine, of a type well known in the art and more fully described below. This machine is frequently used to clean loosely adherent sand from foundry castings. However, this use does not entail the removal of a hard material, firmly attached to the iron, from minute fissures and crevices such as is involved in the process of this invention to remove almost all of the slag from blast furnace slag iron pieces.